1. Field of the Invention
The present invention relates to a filter catalyst which removes particulates contained in an exhaust gas exhausted from an internal-combustion engine such as a diesel engine, and which purifies HC, CO, NOx simultaneously.
2. Description of the Related Art
In the exhaust gas exhausted from the internal-combustion engine such as the diesel engine etc., components such as HC, CO, NOx etc. as well as particulates are contained. The particulates contain substance which is harmful to a human body and which requires to be removed from environmental aspect.
For purification of such exhaust gas, a filter catalyst which is sometimes called as diesel particulate filter (DPF) were used. The filter catalysts are set forth in Japanese Unexamined Patent Publication (KOKAI) No. 9-173,866 and Japanese Unexamined Patent Publication (KOKAI) No. 2002-295,228. In conventional filter catalysts, a catalytic layer was formed on a surface of a catalyst-supporting substrate. That is, the catalyst-supporting substrate made of a porous ceramic has plural cells and cellular wall provided with multiple continuous pores. The catalytic layer is composed of a loading layer made of a heat-resistant inorganic oxide such as alumina etc., and a catalytic metal loaded on the loading layer.
When the exhaust gas passes through ventilation holes formed by the multiple continuous pores of the catalyst-supporting substrate of which surface is coated by a catalytic layer, the filter catalyst captures the particulates and purifies the harmful components except for the particulates. Also, the catalytic layer formed on the catalyst-supporting substrate made of the porous ceramic decomposes the particulates.
If the ventilation holes sufficiently allowing the exhaust gas to pass through was not formed in the filter catalyst, the captured particulates deposit on the catalyst-supporting substrate to cause a pressure loss of the exhaust gas. For suppressing the pressure loss, forming a uniform catalytic layer on an inside surface of the pores of the catalyst-supporting substrate, was known. In this method, a loading layer of the catalytic layer for loading a catalytic metal thereon is produced by a slurry in which a heat-resistant inorganic oxide having small particle diameter is dispersed.
However, due to small particle diameter of the heat-resistant inorganic oxide, there was a problem that the catalytic layer has a lower gas dispersing property, and can not purify the gaseous component such as HC, CO, NOx etc. in the exhaust gas sufficiently.
To the contrary, when the catalytic layer is produced by the heat-resistant inorganic oxide of large particle diameter, there arose another problem. That is, although the gas dispersing property is increased, the loading layer closes the pores of the catalyst-supporting substrate to raise the pressure loss.